Effects of molecular symmetry on enhanced ionization by intense laser pulses

When molecules are driven by intense laser fields linearly polarized along the internuclear axis, their ionization rate vs the internuclear separation R is known to have a maximum at some critical intermediate separation R_c. We show that such a maximum should occur only in molecules whose highest occupied molecular orbitals (HOMOs) are such that the electron cloud is concentrated along the internuclear axis (i.e., σ-type HOMOs). For molecular systems whose HOMO symmetry prevents electron probability distribution along this axis, we find that the ionization rate increases monotonically with increasing R until saturation occurs. In other words, for such molecular systems, there is no critical R_c at which molecular ionization is maximum.